Australasian Science Magazine

Degassing of CO2 from volcanoes may have played a significant role in climate change during the Earth’s long history. Credit: vadim_petrakov

By Stephen Foley

Carbon accumulated in the Earth’s upper mantle over billions of years may be 130 times more significant in controlling the climate than previously thought.

Continental rifts are long, narrow fracture zones bound by faults along which continents eventually break apart in the plate tectonic process. The leading example on the modern Earth is the East African Rift, which stretches more than 3200 km from Djibouti through Ethiopia, Kenya, Uganda and Tanzania to Malawi. Continental rifts are well-known for their abundance of volcanoes, with Tanzania’s Kilimanjaro and Congo’s Nyiragongo providing well-known East African examples. The latter erupted in 2002, when its lava lake escaped through a flank in the volcano, devastating the town of Goma.

The volcanoes of the East African Rift also provide a long-standing riddle for geologists. Many of them produce rocks that are rich in carbonate, or require carbonate during the melting process in order to form; rare volcanoes such as Oldoinyo Lengai in northern Tanzania even erupt pure carbonate lava flows. The riddle has always been to explain where all this carbonate comes from. Why do rifts contain such concentrations of carbonate-bearing volcanic rocks when other tectonic environments do not?

Rifts are also the sites of massive CO2 degassing along faults and at volcanoes – much higher than in other areas around the world. Research is showing that the solution to the riddle lies in appreciating the depth of geological time, and in understanding the mobility of carbon as diamond and carbonate at depth.

Appreciating Deep Time

Scales in geological time can be problematic for peoples’ comprehension of natural phenomena. This is due to the insignificantly short time of human lives relative to those of many Earth processes. Our recollection of even major events pales after a few years, and societies have collective memories that seem to last about a generation.

For instance, villages are sometimes built within the craters of active volcanoes. The large city of Naples has grown up around the flanks of Vesuvius despite this volcano’s protracted history of devastating eruptions. Likewise the word “tsunami” was essentially unused outside scientific circles before the event in Japan in 2004, but many hundreds of tsunamis have affected civilisations in times now long forgotten by humankind.

But human history and pre-history makes up a miniscule proportion of the age of the Earth. To appreciate the depth of geological time, we can use analogies. If we compress the entire 4,567,000,000 years of Earth history into a single calendar year, then 1 day is equal to 12.5 million years, a minute accounts for 8680 years, and a single second corresponds to 145 years. Continental rifts are usually volcanically active for about 40 million years, which sounds like an eternity yet it corresponds to only 3 days on our metaphorical calendar.

Carbon in the Deep Earth

Over the past few years, the discussion surrounding the role of CO2 in climate change has led scientists to study the cycling of carbon not only in the atmosphere and biosphere but also deep in the Earth’s crust and mantle. The solid Earth has enormous reservoirs of carbon, which is released into the oceans and atmosphere as carbon dioxide in volcanoes, and is returned to the mantle at subduction zones.

Volcanoes that release CO2 are concentrated in ocean ridges and in island arcs above subduction zones. Volcanologists have made many measurements of the output of CO2 here, but attempts to budget carbon cycling throughout the inner Earth have not accounted for the role of continental rifts before now.

It is now known that large amounts of carbon dioxide are lost from volcanoes, even during the long periods when they are dormant. Furthermore, enormous amounts of CO2 are lost along faults at the sides of continental rifts. This is easily overlooked in most places because the gas is lost directly to the atmosphere, but becomes apparent where it bubbles into lakes and ponds.

With recent measurements at this type of location along the East African Rift, coupled with high-pressure experiments on melting in the Earth’s mantle, we are now in a position to attempt a budget – a balance between the carbon released at the surface as CO2 and what is put into the magma source regions by the migration of melts at depths of 100–200 km.

The key to the accumulation of CO2 in the continents is the stability of the lithosphere (the uppermost mantle, directly beneath the crust, that belongs to the tectonic plates) and the depth of geological time. This lithosphere is as old as the crust above it, and much of it formed 2.5–3 billion years ago. This allows an immense period of time for carbon to be transported in or out of it, and to be concentrated beneath the later rifts.

The mantle lithosphere contained very little carbon at the time of its formation, but the movement of seemingly insignificantly small amounts of melt allow carbon to be slowly sponged up at the bottom of the continents over that enormous time frame, to be released whenever rifts form. The 3 billion year history of the continents is equivalent to the time since the end of April in our Earth calendar, and is about 80 times the normal duration of a continental rift.

From high-pressure laboratory experiments of the melting of mantle rocks at temperatures of around 1000°C, we know that small packets of mantle melt contain large amounts of carbon, and that the mobility of carbon depends on its oxidation state. Although it is released as CO2 at the surface, most of it is probably present as solid carbon without oxygen in much of the mantle beneath the continents. This means it exists as graphite at depths down to about 150 km, and as diamond below that.

Our new carbon budget estimate predicts that carbon content equivalent to 1.5–3% CO2 is present at the base of the continental plates, but this will be present as diamonds. Up to 1% of diamond may be typical of mantle rocks at 150–200 km depth beneath the continents, but it is all destroyed when the continents thin to form rifts and begin to break apart. We see it in the continental rifts only as carbonate in igneous rocks or as CO2 released into the atmosphere.

The amount of carbon that can be sequestered by the gradual accumulation from migrating melts is enough to account for the release of 80–100 million tonnes of CO2 every year for the entire 40 million year history of a continental rift. This exceeds the 50 million tonnes of CO2 released in the present East African Rift. This is possible only because of the immense length of geological time over which this accumulation has occurred. In these new carbon budgets, the role of the continents in CO2 release is 130 times more important than previously thought.

Carbon, Rifts and Supercontinent Breakup

Now that carbon budgets for the deep Earth have been calculated, and inputs are known to approximately match outputs for the modern Earth, we can turn our attention to conditions that do not exist on the Earth today. Intriguing time periods are when the continents were collected together, most recently forming the supercontinent Pangaea about 250 million years ago.

These supercontinents eventually break up, and smaller continental blocks disperse. When this happens there is an unusual abundance of continental rifts, and these will tap a correspondingly larger amount of carbon and release it into the atmosphere.

It has now been shown that the total continental rift length in the late Jurassic and early Cretaceous was three times greater than today, and that this correlates with CO2 levels three to five times higher than in our current, globally warmed atmosphere. There seems to be little doubt that the abundance of continental rifts during supercontinent breakup could contribute directly to a warming of the climate by several degrees.

Linking carbon movements between the solid Earth, oceans, biosphere and atmosphere should eventually lead to a more complete picture of the evolution of climate, beginning from times in the Earth’s history when the composition of the atmosphere did not yet consist of 78% nitrogen and 21% oxygen as it does today. We are closer to an explanation of how the current climate came into being.